LOS ANGELES — In the span of American pop music, few performers have gone as unrecognized as the backup vocalists who harmonize and contextualize the songs of many a heralded lead musician. With "20 Feet From Stardom," some of the most notable finally get their due, but more than a tribute, the film is a recognition of the talent and sacrifice that many of these vocalists have invested in often challenging careers.

With a documentary career that's richly profiled the likes of Muddy Waters, Johnny Cash, Hank Williams, the Rolling Stones and Pearl Jam, Emmy-award winning director Morgan Neville brings an impressive wealth of talent and depth of experience to the project, enhanced by the decades-long perspective of producer and former A&M music exec Gil Friesen, who died late last year.

Departing from the nostalgic star-power of his last film, "Troubadours," profiling Carole King and James Taylor, Neville shines a spotlight on the emergence of black background singers, primarily female, in the R&B and rock-and-roll genres in roughly chronological format.

Looking back to the hit careers of Ray Charles, Ike and Tina Turner, and their contemporaries, Neville pinpoints the career launches of a clutch of backup singers who would go on to impressive careers in pop music, including Mable John, Susaye Greene, Merry Clayton, Darlene Love and Lisa Fischer. Many of these performers started out singing in gospel choirs at church as youngsters, perfecting the call-and-response techniques central to background performance, then segueing into professional careers. "God gave me this talent," Love recalls thinking, "and I intend to use it."

As Stevie Wonder remarks in a typically charismatic interview, during the '60s, "People started wanting songs with feeling." As musical styles intensified and the Civil Rights movement, the sexual revolution and women's liberation took hold, vocalists' performances also became more expressive. Love was among the first black artists to introduce a new style of singing and presentation into backup performance, which was primarily white at the time. As a member of the highly prolific backing girl group the Blossoms ("He's a Rebel," "Monster Mash"), she supported major stars like Frank Sinatra and Sam Cooke, before signing with legendary producer Phil Spector during his "Wall of Sound" era.

Spector reportedly thwarted Love's ambitions however, by having her "ghost" perform the vocals of other singers credited with her performances. Nevertheless, her voice was a "sound we tried to capture for many, many years," says Bruce Springsteen in his warm and admiring interview footage.

From her start in gospel singing, Clayton's career had a somewhat more glamorous trajectory, performing for Lynyrd Skynyrd ("Sweet Home Alabama"), Joe Cocker ("Feelin' Alright) and most famously the Rolling Stones. She recalls the late-night recording session featuring her wailing, soulful vocals on 1969's "Gimme Shelter" in a thoroughly absorbing interview, intercut with Mick Jagger speaking about the significance of that rare performance.

Neville unearths a treasure trove of archival TV, concert and film footage featuring many of these vocalists in their heyday, balancing the material with perfectly lit contemporary studio interviews and performances shot in pristine digital cinematography, supplemented by more informal scenes depicting the frequent challenges of these musicians' careers.

At one point in the film, it's notes that much of the pop music that fans love — especially the hooks that get them singing along — is voiced by background singers. Although achieving crossover success, or launching a comeback career, is fraught with both personal and professional pitfalls, these women demonstrate drive.

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